Automated passport kiosks have reduced Customs wait times for many international travelers at Kennedy Airport and Newark Liberty International, but lines are still too long, a new study has found.

Of the 20 longest average wait times at the airports in 2014, 18 occurred at terminals without the kiosks, according to the Global Gateway Alliance, a New York City travelers' advocacy group.

At terminals where the Automated Passport Control devices are available, the time it takes to clear Customs has been cut by up to 20 percent, the report said.

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The group is calling on U.S. Customs and Border Protection to expand use of the kiosks, which were introduced in New York last year.

Passengers slide passports into a scanner in the kiosk, and after answering questions electronically are issued a clearance ticket that allows them to move through the process faster.

"When there aren't automated kiosks, it makes a difference: eight of the top 10 longest waits at JFK and Newark were at terminals without kiosks," alliance chairman Joe Sitt said in a statement.

"The study also affirms what we already know: In order for the kiosks to work, you have to have the manpower to match. In other words, the fewer officers, the longer the lines," he said.

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The alliance is also calling on the federal agency to add more officers at the international terminals to further expedite screening.

According to the study, the average maximum wait time at all terminals to clear Customs at Kennedy is 48 minutes. At Newark, the average maximum wait is 38 minutes.

The longest waits for international passengers arriving at Kennedy are on Saturdays, between 3 and 4 p.m.; at Newark, wait times peak on Mondays between 2 and 3 p.m.

Despite an increase in the number of passengers on international flights, Customs reduced wait times at the nation's top 10 airports an average of 13 percent last year through the use of passport kiosks and other measures, according to spokeswoman Jennifer Evanitsky.

There are 127 kiosks at Kennedy's terminals 1, 4 and 5; and 20 at Newark's terminal C, she said.

Customs is "actively working to recruit and hire the additional 2,000 CBP officers authorized by Congress in 2014," according to a statement.